Methods of scrambling and descrambling television signals to prevent unauthorized viewing of protected material have been proposed almost since the inception of television. Everything from moving mirrors shown in U.S. Pat. No. 2,531,974, to inserted carriers shown in U.S. Pat. No. 4,074,311 have been proposed. Some systems in use today seek to modify the synchronization pulses in the video signal as shown in U.S. Pat. No. 3,924,059 or insert interfering video signals which are subsequently removed in the descrambling device as shown in U.S. Pat. No. 4,112,464 or insert interfering carriers at the transmitter which are subsequently removed in the descrambling device with notch filters as shown in U.S. Pat. No. 4,074,311.
Randomized switching between inverted and non-inverted video is expensive to perform in the decoder function, requiring demodulation of the received coded signal, and in the case of a decoder which normally is placed on the top of a video set, the remodulation of the signal after it has been restored to viewable condition. While the systems using injection of interfering carrier systems do not require demodulation and remodulation, they have other disadvantages. For instance, such systems can be easily defeated by persons with only modest means and effort. Also, extra transmitter power must be devoted to the interfering carrier. This is detrimental in cable systems, not so much for the slight penalty in consumed energy, but to the care and expense that is involved in maintaining linearity in all the amplifiers that must amplify the signals. The linearity problem becomes especially severe in over-the-air transmission systems because of the high transmitter power involved.
Further, synchronization pulse modification or suppression systems techniques are easy to defeat. On the other hand these techniques lead to very effective scrambling and good quality pictures after descrambling. By combining this technique with the present invention a scrambling-descrambling system is obtained that is both secure and low in cost and delivers good quality descrambled video signals.